The present invention relates to the characterization of a frequency response for a device, and more particularly to the characterization of a frequency response for the frequency translation device over a specified wide bandwidth at multiple center frequencies covering a wide system bandwidth.
Frequency translation devices, such as mixers that combine an input signal with a frequency from a local oscillator to translate the input signal to a different frequency, are important components in radio frequency (RF) systems. The conversion by the mixers of RF power at one frequency into power at another frequency makes signal processing at a receiver or signal generating at a transmitter easier and more efficient. For modern RF systems minimizing linear distortion along a signal path, including contributions from the frequency translation devices, is critical to meet signal quality requirements, such as Error Vector Magnitude (EVM) for communication systems.
In an RF device the mixer is just one building block in any signal path. Other cooperating circuits need to be considered since the mixer interacts with both the preceding and subsequent circuits in the signal path, as well as local oscillator (LO) driving circuits. All mixers are not fully isolated, leading to leakage or feed-through between mixer ports. Also mixers are non-linear devices which inevitably result in inter-modulation distortion. In traditional narrow bandwidth systems, such as 1-10 MHz intermediate frequency (IF) bandwidth systems, the mixers may be approximated as having ideal frequency responses, i.e., flat gain and linear phase over the IF bandwidth, without causing too much degradation in system performance. RF devices, such as traditional spectrum analyzers, make this assumption and only perform channel calibration and alignment at each center frequency. The differences in frequency responses over the IF bandwidth between the center frequencies are ignored. This alignment technique works reasonably well for narrow bandwidth systems. However for wide bandwidth systems, such as RF devices having signal paths with an IF bandwidth of 100 MHz and greater, the interaction between the mixer and cooperating circuits gets more complicated and worse. Considerable errors occur if the mixer response over the bandwidth of interest is assumed to be the same for every center frequency. This results in distorted signals being emitted by transmitters and/or errors being generated in receivers. In test equipment, such distortions or errors may result in the inability to reliably determine whether or not an RF device being tested meets required specifications.
What is desired is a method of characterizing the frequency response of a frequency translation device over a specified wide bandwidth in conjunction with cooperating circuits for a wide system bandwidth in order to provide greater accuracy for RF devices using the frequency translation device.